It is known that 3',5' cyclic purine ribonucleotides are produced in vivo in living animals, including man, and that cellular levels of certain ones of them such as cyclic guanosine monophosphate (C-GMP) and cyclic adenosine monophosphate (C-AMP) are regulated by specific phosphodiesterases. The biological activity of these cyclic nucleotides naturally follows from such in vivo production and regulation. Indeed, as reported in Sutherland, et al., "Cyclic Amp" Am. Rev. Biochem. 37, 149 (1968), cyclic AMP has now been established as an intracellular "second messenger" mediating many of the actions of a variety of different hormones.
According to the "second messenger" theory, first hormone messengers influence adenyl cyclase contained at or within cell walls to intracellularly form C-AMP from adenosine triphosphate upon receipt of the extracellular hormone signal. The formed C-AMP in turn stimulates intracellular functions particular to the target cells of the hormone. C-AMP has been shown to "activate" protein kinases which in turn occasion physiological effects such as muscle contraction, glycogenolysis, steroidogenisis and lipolysis. As a specific example of mediation of steroidogenesis by C-AMP can be mentioned cellular biosynthesis and excretion of corticosteroids as occasioned by C-AMP formed by adenyl cyclase within the cell walls of the adrenal cortex upon receipt of an extracellular signal carried by the peptide hormone ACTH.
In addition to the foregoing and as representative of the diverse roles played by C-AMP in biological processes can be mentioned implication of C-AMP as a participant in or mediator of the following metabolic reactions or pharmacologic agents: glucagon, vasopressin, lutenizing hormone, thyroid-stimulating hormone, insulin, UDPG-.alpha.trans-glucosylase, phosphofructokinase, tryptophan pyrrolase, ketogenesis, amino acid uptake into liver proteins, acetate incorporation into fatty acids and cholesterol of liver conversion of lactate to glucose (gluconeogenesis), release of amylase, water and ion permeability, sugar transport, acid secretion in the gastric mucosa, platelet aggregation inhibition, catabolite repression, potentiation of antiviral activity of interferon, inhibition of HeLa and strain L cells in culture, and stimulation of antibody production (immunologic mechanism).
The so-called adrenergic effects of many hormones and drugs has now been attributed to the intracellular effects of cyclic AMP whose concentration is controlled by adenyl cyclase and cyclic nucleotide phosphodiesterase. Recent investigations have shown that at least part of the physiological effect of cyclic AMP is a result of the activation of specific protein kinases by cyclic AMP as, for example, in neurotubules isolated from the central nervous system.
Corollary to increasing recognition of the role played by this cyclic purine nucleotide has come the suggestion that it be administered in aid of lagging cellular processes. As one example can be mentioned the report that asthma may be caused by a genetic deficiency of adenyl cyclase. A consequence of such deficiency, of course, is a diminished capacity to intracellularly convert ATP to cyclic adenosine monophosphate.
Phosphodiesterase enzymes degrade purine nucleotides such as C-GMP and C-AMP. In the latter case the enzyme catalyzes hydrolysis of the 3',5'-cyclic adenosine monophosphate to 5'-adenosine monophosphate with consequent loss of function. A need has existed for cyclic purine nucleotide analogs which, while retaining the biological activity of the naturally occurring nucleotides, are resistant to degradation by phosphodiesterase. The availability of such C-AMP analogs, for example, could permit maintenance of desired cyclic nucleotide monophosphate levels at dosages reduced from those required with C-AMP itself. Furthermore, the differing specificity of the phosphodiesterase toward cyclic nucleotides of widely varying structure, could enhance the utility of compounds which exhibit different susceptibilities to diesterases of widely varying specificities.
The above notwithstanding, in certain cases it would appear that adenyl cyclase produces harmfully high intracellular levels of cyclic AMP. For example L.C. Chen, et al. in The Lancet, p. 939 (May 8, 1971) demonstrate excessive cyclic AMP production by adenyl cyclase to underly the debilitating dehydration associated with cholera. Further, there is substantial evidence that the positive inotropic and chronotropic effects of catecholamines on the heart are mediated by adenyl cyclase stimulation (S. E. Epstein et al., Annals Int. Med. 72:561-568, (1970 ). Hence, compounds which inhibit adenylate cyclases may act to lower heart rate and be of value in the prevention of arrythmias. The finding that the mitogen, phytohemagglutinin, stimulates the adenylates cyclase of human peripheral blood lymphocytes (J. W. Smith et al., J. Clin. Invest. 50:432-441, 1971) suggests that adenylate cyclase inhibitors might also be useful as immunosuppressive or anti-inflammatory agents.
From all of the foregoing, it appears that advantage would be gained by obtainment of cyclic AMP analogs equal to or superior to cyclic AMP from the standpoint of kinase activity, superior from the standpoint of resistance to phosphodiesterase, and yet capable of inhibiting adenyl cyclase activity. By such means, intracellular cyclic AMP levels could be controlled by adenyl cyclase inhibition, while overall cyclic AMP-dependent kinase activity could be controlled by regulating dosage of the said analog.